The invention relates to an electric lamp comprising:
a glass lamp vessel sealed in a gastight manner and having an SiO2 content of at least 95% by weight;
an electric element arranged in the lamp vessel;
current feedthroughs projecting from the lamp vessel to the exterior and connected to the electric element, comprising:
a metal shell having a gastight bottom and an open end opposite the bottom, and a raised gastight wall surrounding an axis and having an inner surface;
an internal current conductor having an end piece which is connected in a gastight manner to the bottom, which internal current conductor extends from the bottom to beyond the open side of the metal shell and into the lamp vessel and is connected to the electric element, one end of the lamp vessel having a direct gastight fuse with the metal shell.
A lamp of this type is known from DE 15 89 309. The known lamp has a current feedthrough in which the raised circular gastight wall on the open end of the metal shell has a knife edge. The shell is intended to be embedded with its entire raised wall in the quartz glass of the lamp vessel. The embedded raised wall has formed a gastight fuse with the quartz glass so that the gastightness of the feedthrough of the lamp is achieved. The raised wall is surrounded both on its inner side and its outer side by quartz glass which is fused with substantially the complete inner surface and with a part of the outer surface located proximate to the knife edge.
When manufacturing the known lamp, a seal is made enclosing the metal shell in the quartz glass of the lamp vessel. At the location where this seal is to be created, the quartz glass is caused to soften in the presence of the metal shell, the internal current conductor and an electric element, whereafter cooling takes place. The raised wall of the metal shell shrinks due to its relatively high coefficient of expansion (approximately 50*10xe2x88x927 Kxe2x88x921) more strongly than the quartz glass in which it is embedded, while the coefficient of expansion of the quartz glass is approximately 6*10xe2x88x927 Kxe2x88x921. Since the raised wall of the metal shell is provided with a knife edge, no capillary formation will occur due to deformation of the relatively mechanically weak knife edge, which deformation also obviates the occurrence of too high stresses in the quartz glass. There is no fuse of the quartz glass with the internal current conductor which is therefore not embedded in the quartz glass.
A drawback of the known feedthrough is that the knife edge is mechanically weak and can easily be damaged. The risk of a premature end of the life due to too high stresses in the quartz glass and/or the absence of a gastight fuse between the metal shell and the quartz glass has thus increased considerably. A second drawback is that the method of manufacturing the known lamp is relatively cumbersome.
According to the invention, the metal shell encloses the end of the lamp vessel, the metal shell being free from a knife edge.
The lamp according to the invention can be manufactured in a simple manner. Since the metal shell is free from a knife edge, the manufacture of the shell is considerably simplified, for example, by deep drawing from a metal plate and, moreover, the edge of the metal shell is not vulnerable. The risk of damage is thus reduced when inserting the end of the lamp vessel into the shell and, consequently, the risks of problems in the formation of a direct, gastight fuse with the metal shell are also reduced. After heating, the end of the lamp vessel inserted into the metal shell is directly fused in a gastight manner with the inner side of the raised wall and with the internal current conductor. Since the metal shell is not completely embedded in the glass of the lamp vessel but encloses its end and is directly fused in a gastight manner exclusively with its inner side of its raised wall to the glass of the lamp vessel, the manufacture of the lamp is further simplified. The fuse is achieved by, for example, high-frequency heating of the metal shell with the end inserted into it to a high temperature, for example approximately 2000xc2x0 C., in an inert or slightly reducing atmosphere. As a result of a relatively great difference in coefficients of expansion of the quartz glass and the internal current conductor embedded therein, a capillary is present in the cooled state around the internal current conductor. However, for the same reason of a relatively large difference in coefficients of expansion, the end of the lamp vessel in the cooled state is surrounded in a clamping manner by the raised wall of the metal shell so that the direct, gastight fuse of the metal shell with the lamp vessel is improved.
In one embodiment, the metal shell and/or the internal current conductor of the lamp comprise tungsten and/or molybdenum. For establishing the direct, gastight fuse between the metal shell and the quartz glass at a high temperature, the quartz glass must be fluid but the metal shell as well as the internal current conductor must keep their shape. An internal current conductor and a metal shell comprising tungsten, molybdenum and/or mixtures thereof have been found to be eminently suitable for this purpose.
In another embodiment, the metal shell of the lamp comprises a percentage by weight of at most 10% of an additive. Since the dope has been added to the shell, there is not only a physical adhesion but also a chemical adhesion between the metal shell and the quartz glass. Consequently, the adhesion between the metal shell and the quartz glass is improved so that the direct, gastight fuse is further improved.
In a further embodiment of the lamp, the additive comprises an element chosen from the group of rhenium, thorium, yttrium, lanthanum. It has been found that these elements added as additives to the metal shell lead to a very good adhesion between the metal shell and the quartz glass. The direct, gastight fuse is even further enhanced thereby. The additive may be present as a coating on the current feedthrough but may be alternatively distributed in a homogeneous manner in the mass of the current feedthrough.
In a further embodiment, the metal shell in the lamp is provided with an external current conductor on a side of the bottom facing away from the open side, in which the external current conductor may be provided with coupling means. The external conductor can then be easily welded or, if it is provided with, for example, a screw thread, it can easily be connected or screwed to external electric contacts.
In yet another embodiment of the lamp, a current conductor comprising the internal current conductor and the external conductor is passed in a gastight manner through an aperture in the bottom. To this end, the current conductor is connected by means of a gastight welding/soldering compound to the bottom through which it is passed. Since the internal current conductor and the external current conductor consist of one piece, this construction has the advantage that it can be manufactured in a relatively easy way and is relatively strong.
A feedthrough is known from GB 469 978 in which the quartz glass is adhered to the metal shell by means of a sintered coating of finely divided refractory metal which is provided on the quartz glass. This has the drawback that the manufacture of the feedthrough is relatively cumbersome because a coating must be provided, and that the manufacture should take place carefully because the quartz body must maintain its shape. Moreover, the quartz glass has no direct fuse with the metal shell, which may be detrimental to the gastightness of the connection between the metal shell and the quartz glass.
The electric element may comprise, for example, a filament body, in which case the lamp vessel may comprise a halogen-containing filling with a rare gas. On the other hand, the electric element may also comprise an electrode pair. In that case, the lamp has an ionizable filling, for example a filling of rare gas such as xenon, for example, at a pressure of several, for example 7, bar outside the operating condition, and one or more metal halides, possibly with mercury.
The lamp may be incorporated in a reflector which may be closed with a front glass, for example a shield or a lens. The lamp vessel may be covered with an interference filter. The lamp vessel may have an outer, for example quartz glass, envelope which may be connected to the lamp vessel and the envelope may be, for example, UV absorbing.
The electric lamp may be provided with, for example, a set of plates on which one or more feedthroughs are provided, or the feedthroughs may be arranged, for example, opposite each other.